1. Field of Invention
The present invention relates in general to the field of electronic packaging. More particularly, the present invention relates to a polymer conformal coating which contains a phosphine compound that provides corrosion protection for metal conductors electrically connecting one or more electronic components (e.g., the discrete gate resistors in a resistor network array) mounted on a substrate such as a printed circuit board.
2. Background Art
Electronic components, such as microprocessors and integrated circuits, are generally packaged using electronic packages (i.e., modules) that include a module substrate to which one or more electronic component(s) is/are electronically connected. A single-chip module (SCM) contains a single electronic component such as a central processor unit (CPU), memory, application-specific integrated circuit (ASIC) or other integrated circuit. A multi-chip module (MCM), on the other hand, contains two or more such electronic components.
Generally, each of these electronic components takes the form of a flip-chip, which is a semiconductor chip or die having an array of spaced-apart terminals or pads on its base to provide base-down mounting of the flip-chip to the module substrate. The module substrate is typically a ceramic carrier or other conductor-carrying substrate.
Controlled collapse chip connection (C4) solder joints (also referred to as “solder bumps”) are typically used to electrically connect the terminals or pads on the base of the flip-chip with corresponding terminals or pads on the module substrate. C4 solder joints are disposed on the base of the flip-chip in an array of minute solder balls (e.g., on the order of 100 μm diameter and 200 μm pitch). The solder balls, which are typically lead (Pb)-containing solder but may be lead-free solder (e.g., Sn—Ag—Cu solder), are reflowed to join (i.e., electrically and mechanically) the terminals or pads on the base of the flip-chip with corresponding terminals or pads on the module substrate.
Typically, a non-conductive polymer underfill is disposed in the space between the base of the flip-chip and the module substrate and encapsulates the C4 solder joints. The C4 solder joints are embedded in this polymeric underfill and are thus protected from corrosion caused by moisture and carbon dioxide in the air, as well as octanoic acid outgassed from components within the module. However, the use of the polymeric chip underfill may not adequately protect the C4 solder joints from corrosion caused by sulfur components (e.g., elemental sulfur, hydrogen sulfide, and/or sulfur oxides) in the air, especially if the C4 solder joints contain silver as a significant constituent.
It is also known to cover the C4 solder joints with a conformal coating, such as a cast polymer barrier layer, to protect the C4 solder joints from corrosion caused by moisture and carbon dioxide in the air, as well as octanoic acid outgassed from components within the module. For example, U.S. Patent Application Publication No. 2007/0257091 A1, entitled “Chip Module having Solder Balls Coated with a Thin Cast Polymer Barrier Layer for Corrosion Protection and Reworkability, and Method for Making Same” and published on Nov. 8, 2007, to Kuczynski, discloses a thin cast polymer barrier layer covering solder joints that not only protects the solder joints from corrosion caused by moisture and carbon dioxide in the air (as well as octanoic acid outgassed from components within the module), but also provides reworkability. The cast polymer barrier layer is selected from suitable polymers including polystyrene; poly(oxymethyleneoxyethylene); poly(oxybutylethylene); poly(vinylidene chloride); poly(perfluoro-4-chloro-1,6-heptadiene); poly(methacrylic acid), ethyl ester; poly(methacrylic acid), n-propyl ester; poly(methacrylic acid), i-propyl ester; poly(methacrylic acid), n-butyl ester; poly(methacrylic acid), i-butyl ester; poly(methacrylic acid), sec-butyl ester; poly(methacrylic acid), n-amyl ester; poly(methacrylic acid), i-amyl ester; poly(methacrylic acid), 1,2-dimethylpropyl ester; poly(methacrylic acid), neopentyl ester; poly(methacrylic acid), 3,3-dimethylbutyl ester; poly(methacrylic acid), 1,3-dimethylbutyl ester; poly(perfluoropropylene); poly(vinyl alcohol); poly(vinyl butyrate); poly(methyl isopropenyl ketone); and combinations thereof. Unfortunately, like the polymeric chip underfill, the use of such a conformal coating may not adequately protect the C4 solder joints from corrosion caused by sulfur components (e.g., elemental sulfur, hydrogen sulfide, and/or sulfur oxides) in the air, especially if the C4 solder joints contain silver as a significant constituent.
It is also known to use silicon nitride (Si3N4) to seal a flip-chip. For example, U.S. Pat. No. 6,972,249 B2, entitled “Use of Nitrides for Flip-Chip Encapulation” and issued Dec. 6, 2005 to Akram et al., discloses a semiconductor flip-chip that is sealed with a silicon nitride layer on an active surface of the flip-chip. The silicon nitride layer covers the chip active surface, including the bond pads and conductive connectors such as solder balls formed over the bond pads. Unfortunately, while the silicon nitride layer may provide some degree of protection for the bond pads and conductive connectors from corrosion caused by sulfur components (e.g., elemental sulfur, hydrogen sulfide, and/or sulfur oxides) in the air, the silicon nitride layer is difficult to process and work with.
The problem of corrosion caused by sulfur components (e.g., elemental sulfur, hydrogen sulfide, and/or sulfur oxides) in the air is especially severe when one or more of the metal conductors that electrically connect an electronic component is/are a silver-containing metal. For example, each of the gate resistors of a resistor network array typically utilizes a silver layer at each of the gate resistor's terminations. Gate resistors are also referred to as “chip resistors” or “silver chip resistors”. Typically, gate resistors are coated with a glass over coat for corrosion protection. Also for corrosion protection, it is known to encapsulate gate resistors in a resistor network array by applying a coating of a conventional room temperature-vulcanizable (RTV) silicone rubber composition over the entire printed circuit board on which the resistor network array is mounted. However, the glass over coat and conventional RTV silicone rubber compositions fail to prevent or retard sulfur components in the air from reaching the silver layer in gate resistors. Hence, any sulfur components in the air will react with the silver layer in the gate resistor to form silver sulfide. This silver sulfide formation often causes the gate resistor to fail, i.e., the formation of silver sulfide, which is electrically non-conductive, produces an electrical open at one or more of the gate resistor's terminations.
FIG. 1 illustrates, in an exploded view, an example of a conventional gate resistor 100 of a resistor network array. A resistor element 102 is mounted to a substrate 104, such as a ceramic substrate. The gate resistor 100 includes two termination structures 110, each typically comprising an inner Ag (silver) layer 112, a protective Ni (nickel) barrier layer 114, and an outer solder termination layer 116. Typically, for corrosion protection, the gate resistor 100 is coated with a glass over coat 120. Additionally, for corrosion protection, a coating (not shown) of a conventional RTV silicone rubber composition may encapsulate the gate resistor 100. As noted above, it is known to encapsulate gate resistors in a resistor network array mounted on a printed circuit board by applying a coating of a conventional RTV silicone rubber composition over the entire board. However, as noted above, the glass over coat 120 and conventional RTV silicone rubber compositions fail to prevent or retard sulfur components in the air from reaching the inner silver layer 112. Hence, any sulfur components in the air will react with the inner silver layer 112 to form silver sulfide 202 (shown in FIG. 2). FIG. 2 illustrates, in a sectional view, the conventional gate resistor 100 shown in FIG. 1, but which has failed due to exposure to sulfur-bearing gases. The silver sulfide formation 202 (often referred to as silver sulfide “whiskers”) produces an electrical open at one or more of the gate resistor's terminations 110 because silver sulfide is an electrical non-conductor and, thereby, results in failure of the gate resistor 100.
The use of silver as an electrical conductor for electrically connecting electronic components is increasing because silver has the highest electrical conductivity of all metals, even higher than copper. In addition, the concentration of sulfur components in the air is unfortunately increasing as well. Hence, the problem of corrosion caused by sulfur components in the air is expected to grow with the increased use of silver as an electrical conductor for electrically connecting electronic components and the increased concentration of sulfur components in the air.
U.S. Pat. No. 7,553,901 B2, entitled “RTV Silicone Rubber Composition for Electric and Electronic Part Protection, Circuit Boards, Silver Electrodes, and Silver Chip Resistors” and issued on Jun. 30, 2009 to Horikoshi et al., discloses RTV silicone rubber compositions comprising an organopolysiloxane, an organosilicon compound or partial hydrolytic condensate thereof, and a non-aromatic amino-bearing compound. The compositions are purported to prevent or retard electronic parts encapsulated or sealed therewith from corrosion with sulfur-containing gas. However, the amino-bearing compound in the compositions binds not only with sulfur components in the air but, disadvantageously, binds with carbon dioxide in the air. Hence, the amino-bearing compound in the compositions is quickly consumed by carbon dioxide in the air and is not available to bind with sulfur components in the air. The amino-bearing compound in the compositions also disadvantageously binds to tin catalyst, which is typically required in the formation of RTV silicone rubber compositions. In addition, the amino-bearing compound, though non-aromatic, is nonetheless not completely non-volatile because it is not bonded directly to the polysiloxane backbone.
Therefore, a need exists for an enhanced composition, method and apparatus for protecting metal conductors for electrically connecting electronic components from corrosion caused by sulfur components (e.g., elemental sulfur, hydrogen sulfide, and/or sulfur oxides) in the air.